The present invention relates to a system for recording/reproducing information to/from a rewritable compact optical disc, and more particularly to a compact magnetooptical disc system for information recording/reproducing with a 2.5 inch magnetooptical disc cartridge for example.
Rewritable magnetooptical discs are used nowadays for computer data recording, document files and the like. In the following description, although a magnetooptical disc is used by way of example, other rewritable optical discs may be used as well. A rewritable optical disc cartridge used in the present system is constructed of an optical disc for recording/reproducing with optional erasure and rewriting, and a plastic case for housing the disc therein to facilitate its handling and detachable mounting to a recording/reproducing system. A rewritable optical disc has various sizes ranging from a large 12 inch type, 5.25 inch type to a small 3.5 inch type. Recently a 3.5 inch type disc has drawn attention as electronic apparatus tends to become more compact.
A conventional rewritable optical disc will be described in detail by using a 3.5 inch magnetooptical disk by way of example, while referring to FIG. 1. FIG. 1A is a plan view of a magnetooptical disc, and FIG. 1B is a partial enlarged cross section thereof.
As shown in FIG. 1B, a conventional 3.5 inch magnetooptical disc 1 has a circular substrate 1a made of transparent resin such as polycarbonate. There is formed on the substrate la a magnetooptical film (molybdenum (Mo)-film) 1b made of such as Iron (Fe), Cobalt (Co), Terbium (Tb), Gadolinium (Gd) for information recording/reproducing by means of the magnetooptical effect. There is further formed on the magnetooptical film 1b a protection film 1c made of such as ultraviolet hardened resin. At the center of the disc 1, a metal hub 2 is fixedly mounted for driving the disc. This disc is housed, as described previously, within a plastic case (not shown) to form a 3.5 inch magnetooptical disc cartridge.
The outer diameter S1 of the disc 1 is 86 mm. The outer diameter S2 of a record area 3 for information recording/reproducing relative to the magnetooptical film 1b is 80 mm, and the inner diameter S3 thereof is 48 mm. The record width W1 of the record area 3 in the radial direction is therefore 16 mm. Within the record area 3, there are formed 10667 spiral or concentric record tracks at 1.5 micron pitch. Each record track includes data areas for information recording/reproducing and management areas for management information storage, both areas being alternately disposed within the track.
The conventional 3.5 inch magnetooptical disc 1 is used by mounting it on a conventional 3.5 inch magnetooptical disc system shown in FIG. 2 which illustrates the recording/reproducing operation of the system.
As shown in FIG. 2, the conventional 3.5 inch magnetooptical disc system 5 is constructed of the 3.5 inch magnetooptical disc 1, base 4, electromagnet 6, feed device 7, optical recording/reproducing device 10, and so on.
The 3.5 inch magnetooptical disc 1 is rotated at a Constant Angular Velocity (CAV) of 3600 rpm by a spindle motor (not shown) fixed at the base 4, while the magnetooptical film 1b is applied with a bias magnetic field in the perpendicular direction by means of the electromagnet 6 which is disposed on the side of the protection film 1c and spaced therefrom by a gap. A laser beam L1 from the optical recording/reproducing device 10 is applied from the substrate side 1a to the magnetooptical film 1b for information recording/reproducing.
The optical recording/reproducing device 10 is constructed of a fixed unit 11 and an optical head 20. The fixed unit 11 is constructed of a laser diode 12, collimator lens 13, beam splitters 14 and 15, converging lenses 16 and 18, and photodiodes 17 and 19. The optical head 20 is constructed of a prism 21, objective lens 22, and focus servo actuators 23 and 24.
A laser beam radiated from the laser diode 12 is made parallel by the collimator lens 13 and shaped in a predetermined beam cross section by a beam shaping prism (not shown). The shaped laser beam is then applied via the beam splitter 14 to the prism 21. The laser beam L1 reflected by the prism 21 is formed into a light spot 25 of a small diameter by the objective lens 22 and applied to the surface of the magnetooptical film 1b on the record track of the record area 3 of the disc 1. The light spot 25 applied to the surface of the magnetooptical film 1b has a light intensity suitable for recording during a record mode of the device 10 and another light intensity suitable for reproducing during a reproducing mode. The light intensity is controlled by an output control device (not shown) for the laser diode 12.
During the recording mode, digital signals corresponding to the record data are recorded in such a manner that the perpendicular magnetization direction of the magnetooptical film 1b on the record track is reversed at the position where the light spot 25 is applied, with the aid of the bias magnetic field generated by the electromagnet 6.
During the reproducing mode, the rotation direction of the polarization plane of the laser beam reflected at the surface of the magnetooptical film 1b changes with the perpendicular magnetization direction thereof due to the magnetooptical effect. The laser beam with changed polarization plane then returns back to the objective lens 22 and prism 21 and is reflected by the beam splitter 14 and applied via a half-wave plate (not shown) to the polarizing beam splitter 15. The laser beam passing through the polarizing beam splitter 15 is converged by the converging lens 16 and converted into an electric signal by the photodiode 17 to be outputted as a detected signal. The laser beam reflected by the polarizing beam splitter 15 is also converged by the converging lens 18 and converted into an electric signal by the photodiode 19 to be outputted as a detected signal. The magnitude of both detected signals changes oppositely in accordance with the rotation direction of the polarization plane by means of the polarizing beam splitter 15, so that the digital signal can be reproduced in accordance with the difference between both detected signal outputs.
The fixed unit 11 of the optical recording/reproducing device 10 is fixed on the base 4. The optical head 20 is mounted on a movable section 9 of the feed device 7. The movable section 9 of the feed device 7 is moved in the radial direction D1 of the disc 1 by means of a linear motor 8 which is mounted on the feed device 7 fixed on the base 4. As a result, the light spot 25 is allowed to move within the range from the inner diameter circumference to the outer diameter circumference of the record area 3 so that the light spot performs a so-called feed operation to thereby search a record track. The base 4 is adapted to be held in position at a main chassis (not shown) of the device 10. As a countermeasure against surface vibration or the like of the disc 1, a so-called focus servo (in the direction of an arrow D2) for automatically moving the focal point of the light spot 25 on the surface of the magnetooptical film 1 is carried out by controlling the vertical position of the objective lens 22 with a focus servo actuator 23 in accordance with the detected signal outputs.
Furthermore, as a countermeasure against eccentricity of the disc 1, a so-called tracking servo (in the direction of an arrow D3) for automatically making the light spot 25 to trace the record track is carried out by moving the position of the objective lens 22 in the radial direction of the disc 1 with a tracking servo actuator 24 in accordance with a signal output detected from the management area. The range of tracking of the light spot 25 in the radial direction of the disc 1 with the tracking servo has in general a limit of about .+-.1 mm. Therefore, the positioning of the light spot 25 in the radial direction of the disc 1 is carried out precisely by the tracking servo (arrow D3) and coarsely by the feed operation (D1).
If the conventional 3.5 inch magnetooptical disk system 5 constructed as above is mounted in a battery driven portable personal computer which is a typical example of personal use, there is a problem of a large outer dimension of the system 5a, and also of a large weight not suitable for portable use. There is another problem of a large power consumption of a battery.